Reduction of gaseous pollutants in combustion flue gas

ABSTRACT

Fuel is burned in a primary combustion chamber with less than the air required for stoichiometric combustion so that the combustion gases have a high carbon monoxide (CO) and hydrocarbon content and the temperature of the gases is held below that at which significant nitrogen oxides (NO x ) would be produced. The combustion gases are then passed through a secondary combustion zone in which more air is injected into the gas stream to oxidize the CO and hydrocarbons to carbon dioxide (CO 2 ). The secondary burner comprises a plurality of foraminous tubes through which secondary air is emitted. Combustion in the secondary zone is maintained at a temperature below that at which nitorgen oxides (NO x ) will be produced in significant quantities.

This is a division, of application Ser. No. 295,249, filed Oct. 5, 1972and now U.S. Pat. No. 3,837,788.

BACKGROUND OF THE INVENTION

Oxides of nitrogen and carbon monoxide are gaseous pollutant products ofthe combustion of hydrocarbon fuels. As pollution control standardsbecome more stringent, the reduction or elimination of these productsbecomes a serious problem.

SUMMARY OF THE INVENTION

The invention comprises a combustion method and apparatus which ischaracterized by burning carbonaceous or hydrocarbon fuel in a primarycombustion zone with less than the stoichiometric amount of air requiredfor complete combustion. Generally up to about 75 or 80% of thestoichiometric amount is supplied to the primary zone. Incompletecombustion results in the temperature of the combustion gases remainingbelow 2700° F, a temperature above which significant quantities ofNO_(x) would be produced. Incomplete combustion in the primary zoneresults in the gaseous combustion products containing a high percentageof CO, unburned hydrocarbons and carbonaceous materials. All of the hotgases from the primary zone are then passed through a secondarycombustion zone where air is injected in the gas stream for oxidizingthe CO, unburned hydrocarbons and carbonaceous materials to innocuousCO₂ under such conditions that a temperature is never exceeded at whichnitrogen from the air or from the fuel might be oxidized to NO_(x) insignificant quantities.

The secondary combustion zone includes a plurality of foraminous tubesover which the gaseous combustion products exiting the primarycombustion zone are constrained to pass. Air at positive pressure is fedinto the tubes from any suitable source. The tubes have foramina of somekind such as pores or perforations for emitting air into the gaseouscombustion product stream. The secondary air mixes with the gases tosupport a low temperature combustion process which oxidizes the CO,unburned hydrocarbons and carbonaceous materials to CO₂ undertemperature conditions which minimize production of NO_(x).

The invention is further characterized by controlling the total airrequired for combustion of the fuel at prevailing feed rates in theusual way. More specifically, a primary damper is provided forcontrolling air flow to the primary combustion zone. This primary damperoperates coordinately with the fuel feed control device in response tothe thermal demand of the system. A secondary damper is also providedfor automatically regulating the secondary air flow in response to theCO level in the flue or stack gas.

A primary object of this invention is to reduce air pollution byreducing NO_(x), CO, hydrocarbon and particulate content of the exhaustgases from carbonaceous and hydrocarbon fuel burners.

A further object of this invention is to provide a combustion system andmethod in which combustion conditions are so controlled thatconsequential quantities of NO_(x) are not produced, thereby obviatingthe need for removing any NO_(x) from the flue gases.

A still further object is to minimize NO_(x) production withoutadversely affecting the thermal efficiency of the combustion apparatus.

Another object is to provide a device for reducing air pollutants,expecially NO_(x) which device can be readily adapted to various typesof boilers and other fuel burning devices as well.

How the foregoing and other more specific objects of the invention areachieved will appear in the detailed description of an illustrativeembodiment of the invention which will be set forth shortly hereinafterin reference to the drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical side elevation, partly in section, of a boilerincorporating the invention;

FIG. 2 is a front elevation view of the boiler shown in the precedingfigure;

FIG. 3 is a front elevation view of the secondary burner as viewed inthe direction of the arrows 3--3 in FIG. 1;

FIG. 4 is a top view of the secondary burner assembly shown in thepreceding figure;

FIG. 5 is a front elevation view of an alternative type of secondaryburner, with some parts broken away and in section;

FIG. 6 is a transverse section taken on the line 6--6 in FIG. 5; and

FIG. 7 is a section of one of the burner tube assemblies taken on theline 7--7 in FIG. 5.

DESCRIPTION OF A PREFERRED EMBODIMENT

Although the invention is applicable to various fuel burning apparatusit will be discussed for purposes of illustration in connection with asteam or hot water boiler.

FIGS. 1 and 2 show a boiler which is somewhat schematically representedand which incorporates the new pollutant reduction system. Theillustrative boiler comprises a housing 10 in which there is an uppersteam or hot water drum 11 that connects with a lower feed water drum 12by means of a group of water filled tubes 13. The tubes have webs 14welded between them to enlarge the heat absorption surface and toconfine the flue gases to flow in a predetermined path. There is a groupof tubes 13 on the other side of the boiler also extending from upperdrum 11 to lower drum 12. These tubes together with drums 11 and 12define a space in which heat is absorbed by radiation from thecombustion devices and from the hot combustion gases that flow throughthe boiler. Ultimately, the combustion gases reach an adapter 15 fromwhich the gases are piped to a stack, not shown, for discharge to theatmosphere.

The boiler has a primary combustion chamber 20 comprising a cylindricalrefractory shell 21 which is continuous with a conical extension 22 andwhich defines an internal volume 24 which is herein called a primarycombustion zone. Gaseous or vaporized liquid fuel may be burned in theprimary combustion zone. The fuel is injected with a nozzle 25 which hasa pipe 26 leading back to a burner block 27 where a fuel line connection28 is made thereto. Nozzle 25 is supported on its feed pipe 26 centrallywithin a hollow cylindrical element 29 which has a plurality of openings30 that act as a diffuser for air which is supplied for combustion inprimary combustion zone 24. The burner assembly may be any conventionaltype that is suitably adapted for burning gas or liquid fuel.

Supported on the front of the boiler is a motor 31 on whose shaft 32there is mounted a fan 33. In a conventional manner, rotation of fan 33causes generation of pressurized air in a compartment 34. Thepressurized air is supplied both to the primary combustion zone 24 andto a secondary combustion zone 61 and, in some designs in accordancewith the invention, to a tertiary combustion zone, not shown, as will beexplained shortly hereinafter.

In the depicted embodiment, there is a main duct 35 directing combustionair to two subdividing ducts, a primary combustions air duct 41 and asecondary combustion air duct 40. Primary air duct 41 has a damper 46mounted in it for rotation on a shaft 47. Damper 46 may be turned toregulate air flow through primary air duct 41. Shaft 47 is driven by amotor 39 as indicated by the dashed line 38. Also driven by motor 39, asindicated by the dashed line 38', is a cam 36 whose follower 37 operatesa fuel flow control valve, not shown. By conventional means which arenot shown, motor 39 is driven bidirectionally to operate damper 46 andcontrol primary combustion air flow in response to boiler operatingconditions including steam or hot water load demand. Thus, damper 46 andfuel control cam 36 are operated coordinately to maintain the desiredfuel-to-air ratio in the primary combustion zone 24 throughout theentire range of boiler operating conditions. In accordance with theinvention, less than the amount of air for complete combustion isnormally supplied to primary combustion zone 24.

In general, from about 75 to 80% of the air delivered by the fan 33 isfurnished to the primary combustion zone 24 and the balance is furnishedto the secondary combustion device 42 which is in secondary zone 61.Primary air duct 41 leads to a compartment 43 from which air flowsthrough diffuser ports 30 into primary combustion zone 24 where the airenables the fuel injected by nozzle 25 to be burned incompletely, inaccordance with the invention, by suitably predetermining thefuel-to-air ratio throughout the operating range of the boiler.

The smaller secondary air duct 40 has a damper 44 in it which is mountedfor rotation on a shaft 45. Shaft 45 is turnable bidirectionally by amotor 49 in response to a condition such as the CO level which prevailsin the flue gas stack 15 leading from the boiler. Motor 49 is controlledby a servo-controller 50 which in turn responds to CO level in the stackas sensed by a suitable sensor 51. Changes in CO level result insecondary damper 44 altering the amount of air delivered to thesecondary combustion device 42 by way of duct 40.

In one operating mode the secondary air damper 44 may be wide open whenthe boiler is being started. This results in substantially completecombustion in the secondary zone 61 of the residual carbonaceous solids,other particular matter, hydrocarbons and CO which are not completelyoxidized in the primary combustion zone 24. Then the small damper 44 maybe gradually closed until CO is sensed in the stack gas. The system thengoes on automatic operation to maintain the CO level near zero or belowa preset minimum substantially by controlling secondary combustion airflow through regulation of damper 44. The primary combustion air flowis, during normal operation of the boiler, regulated coordinately withthe proper fuel ratio in accordance with the thermal load on the boilerand on other conditions.

The total amount of air supplied for combustion in the primary andsecondary combustion zones is generally slightly greater than thestoichiometric requiresments for complete combustion of the combustiblecomponents of the fuel, but it will be understood that stoichiometriccombustion conditions are not approached in the primary zone 24, inaccordance with the invention, because gas temperature in the primaryzone under these conditions could reach 2,700° F and cause much NO_(x)to be produced which is contrary to the invention. For the purposes ofthe invention, the boiler is operated so that gases in the primarycombustion zone are maintained well below 2,700° F and some incompletelyoxidized products result.

In an alternative form of the invention, a main damper, not shown, isinstalled in main duct 35 preceding the ducts 40 and 41. This maindamper may be driven by motor 39 which also drives fuel control cam 36and damper 46 in response to boiler demands as in the illustratedembodiment. The smaller secondary air control damper 45 is then used forfine control in response to CO level in the stack gas. Control over thecomposition of the effluent combustion products may also be achievedwith another alternative in which the main damper, not shown, in mainduct 35 and the fuel control cam 36 are controlled by motor 39 inresponse to demand on the boiler while the primary damper 46 in duct 41and secondary damper 45 in duct 40 are jointly controlled by CO levelresponsive motor 49.

Attention is now invited to FIGS. 1, 3 and 4 for a more detaileddescription of the new secondary combustion device 42. As indicatedheretofore, the secondary combustion device is situated in a secondarycombustion zone 61 at the outlet end of the primary combustion chamber20 so that all gases of combustion must flow through or near the device42. Basically, the secondary combustion device 42 comprises two parallelrows of foraminous tubes, the tubes in one row being marked 54 and thetubes in the other row being marked 53. The purpose of the tubes is todiffuse or inject secondary combustion air uniformly into the stream ofgaseous combustion products flowing from primary combustion chamber 20to promote mixing and insure complete combustion without an excessiveamount of secondary air. Thus, in this embodiment, a tube such as 54 isprovided with two longitudinally extending rows of small holes 55 and 56through which air may emerge into the gaseous combustion product stream.In this case, the tubes are supported in a header 57 over which there isa cap 58 to prevent evolution of undispersed air from the ends of thetubes into the gas stream. The lower ends of the tubes are also in aheader 59 but the lower ends of the tubes communicate with the secondaryair duct 40 which is under control of small damper 44. Air evolving fromtubes 53 and 54 through the rows of small holes 55 and 56 effectuatescombustion of CO and unburned substances such as hydrocarbons emanatingfrom the primary combustion chamber. The tubes are mounted in asupporting structure 60 which maintains their position in the gaseouscombustion product stream. The temperature in the secondary combustionzone is dependent upon the temperature and quantity of air dischargedfrom tubes 53 and 54 and the radiation of heat away from this zone. Theboiler tubes to which the secondary combustion device 42 is exposedinsures that the secondary combustion occurs at a temperature of 2,500°F or below which is low enough to minimize oxidizing nitrogen from thefuel or the combustion air to nitrogen oxides. In this embodiment, tubes53 and 54 may be comprised of a suitable refractory or stainless steelor other material which will not degrade at prevailing temperatures. Itis desirable to locate and arrange the secondary combustion device 42 insuch manner that it can radiate heat to the boiler tubes so that thesecondary combustion air flowing through the perforated tubes will notbe significantly preheated before it emerges.

In FIG. 4, the rows of holes 55 and 56 are circumferentially spacedapart so as to intercept a central angle of about 120°. It will beunderstood that the rows of holes may be angularly closer or fartherfrom each other as well without defeating the purposes of the invention.An angle of about 120° is, however, desirable since it enhancesturbulence and mixing of the injected air and stream of gaseouscombustion products in which case more complete combustion is promoted.With this angle, turbulence and good mixing are obtained because the airemitted from the small holes at just about the point where the gasstream flowing past the tubes begins to separate therefrom to formvortices.

Various kinds of hollow air dispersing means may be substituted for theperforated metal tubes 53 and 54 which were described. For instance, thehollow means may be made of sintered metal or ceramic or otherrefractory which is perforated or porous partially or entirely aroundtheir perimeters. The means may also be provided with narrow continuousor interrupted longitudinal slots for emitting air instead of beingprovided with many small holes or pores. The tubes may be made of anymaterial that withstands the conditions that prevail in the vicinity ofthe secondary combustion device, 42. There may also be more or fewer airsupply tubes in a row or more or fewer than the two rows illustrateddepending upon requirements of the system.

A modified form of secondary combustion device will now be described inreference to FIGS. 5-7. This embodiment is distinguished by its havingmeans for keeping the air emitting tubes cool and for precooling or, atleast preventing, preheating of the incoming secondary air.

In FIG. 5, the modified secondary combustion device is generallydesignated by the reference numeral 70. It comprises a frame 71 whichsupports a water feed header 72 and a water discharge header 73. Beneaththe lower water header 72 is a secondary air feed header 74 that issupplied through secondary air duct 40 under the control of damper 44.As can be seen in FIG. 6, in this exemplary embodiment there are againtwo rows of water cooled, air emitting tube structures, the structuresin the back row being marked 75 and those in the front row 76. As in theprevious embodiment, it will be understood that the number of tubestructures in each of the rows and their size and geometry will dependon the gas quantities handled in a particular boiler size.

One of the air emitting tube structures 75 will be described since theymay all be the same. Referring to FIG. 5, one may see that the structure75 comprises a central secondary air conducting tube 80 which has twolongitudinally extending rows of small holes such as 81 and 82 throughwhich secondary combustion air may emerge into the gaseous combustionproducts stream. As can be seen particularly well in the cross sectionalview of one of the tube structures 75 in FIG. 7, the rows of holes 81and 82 are aligned with angularly diverging longitudinally disposedhollow flutes 83 and 84, respectively. Both flutes have the samegeometry. For instance, flute 84 extends radially from tube 80 and has alongitudinally extending open ended slot 85 which conducts the secondaryair emitted through the row of holes 82 to the gaseous combustionproduct stream surrounding the secondary combustion device. As can beseen in FIG. 7, the tips of the slotted flutes are beveled so that theslots 85 open substantially exclusively on the leeward side of gaseouscombustion product flow.

The center tube 80 in this embodiment is capped at its upper end 86 soas to constrain all of secondary combustion air to flow through theorifices or small holes 81 and 82. The lower ends of center tubes 80 areconnected with header 74 through which secondary combustion air issupplied through duct 40 under the control of small damper 45. The innertube is surrounded by a concentric outer tube 87 which defines a waterjacket 89 around the inner tube. Water flows axially in the segments ofthe jacket between the flutes 83 and 84. This results from the fact thatthe lower end 88 of outer tube 87 connects into water feed head 72 as isparticularly evident in FIG. 5 in the lower broken away portion of thetube structure. The inner and outer tubes are suitably welded orotherwise sealed where they pass through or into their respectiveheaders. Water flowing axially through the water jacket area 89 emergesat the top end of the structure and continues its flow path through acavity 90 and through a hole 91 in upper water exit header 73. Ofcourse, the inner air tube 80 may be adapted to extend through upperwater exit header 73 to another secondary air header, not shown, or theair inlet header may be arranged to feed the tubes 80 from the topinstead of the bottom or there may even be two independent air headersfeed tubes from the top and bottom. The design will depend on thequantity of primary gaseous combustion products to be burned and uponmeeting the condition that secondary combustion should be well below2,700° F such as at about 2,100° F to oxidize the CO and hydrocarbonscompletely and yet inhibit NO_(x) production.

In FIG. 7, an incremental filament of the gaseous combustion productstream is indicated by the arrowed line 95. It will be noted that thistypical incremental stream deflects off of the periphery of the outertube 87 at an angle such that the stream will intersect with thesecondary air stream emerging from the slotted flutes 83, 84 in whichcase turbulence is maximized. This promotes oxidation of the residualhydrocarbons and carbon monoxide and any other burnable matter in thegaseous combustion product stream from the primary combustion chamber20. The fact that the tube structure is water cooled not only preventsits thermal degradation but it also results in precooling of theincoming secondary combustion air which aids in suppressing thetemperature of the secondary combustion products to well below the2,700° F at which nitrogen oxide might be formed. It is also desirableto position the secondary combustion device in relation to the heatabsorbing surfaces of the boiler such that the device will be cooled byradiating to the surfaces. The existence of a fairly high CO level inthe secondary combustion gases also tends to inhibit formation ofnitrogen oxide because the air reacts preferentially with the carbonmonoxide rather than with the nitrogen derived from the air or the fuel.

In the previously described embodiments, there are primary and secondarycombustion zones. For example, in the secondary combustion device 42shown in FIGS. 1, 3 and 4 the rows of foraminous tubular elements 53 and54 are both supplied from the same secondary air duct 40 in which casethere is actually a single secondary combustion zone in the vicinity ofdevice 42. Similarly, in the FIGS. 5-7 embodiment, the foraminous tubessuch as 80 are all connected into a common header 74 which is suppliedfrom the secondary air duct 40 in which case all of the tubes contributeair to a single secondary combustion region. Although specific structureis not shown, those skilled in the art may readily infer from what hasbeen disclosed that a tertiary combustion zone may also be provided.This can be done by connecting the leading rows of tube structures 54 or75 in the FIG. 1 or FIG. 6 embodiments, respectively, to one secondaryair supply. The other rows of tubes 53 or 76 in the respectiveembodiments, may then be connected into a third pressurized air supply,not shown. Thus, primary, secondary and tertiary combustion zones arecreated. The zone ahead of the rows of tubes 53 or 76 becomes thesecondary zone and the zone in front of the rows of tubes 54 or 75become the tertiary combustion zone. In this arrangement, the gaseouscombustion products which are rich in CO and unburned hydrocarbons fromprimary combustion chamber 20 undergo further burning in two stages inthe secondary and tertiary combustion zones so that the probability ofelevating the gas temperature to that above which NO_(x) might be formedis further reduced. In this arrangement, it is easier to keep the tubescool since the flame surrounding each of the rows is not as intense.

During operation of the illustrated embodiment, changes in the CO levelof the effluent stack gas are sensed and used to control the smalldamper 44 in the secondary air supply duct 40. If CO in the stackincreases, secondary air is increased by automatic increased opening ofdamper 44 in which case the combustion products from the primary zoneare more effectively oxidized in the secondary zone and the CO levelgoes down again. A decrease in stack gas CO level brings about converseaction. The essence of the system is to assure that gases passingthrough the secondary combustion device are oxidized but to minimize theintensity of flame in that region so that the temperature will not beincreased to the point where nitrogen oxides would be produced. Usually,the CO level of the stack gases will reach a steady state as long asload requirements on the boiler are fairly constant. However, if thereis greater or lesser load, the fuel and air to the primary combustionzone change accordingly in which case the CO level in the flue gases maychange and effectuate automatic readjustment of the secondary air.

Although embodiments of the invention have been described inconsiderable detail, such description is to be considered illustrativerather than limiting for the invention may be variously embodied and isto be limited only by interpretation of the claims which follow.

We claim:
 1. Fuel burning method for minmizing the discharge of nitrogenoxide in its exhaust gases comprising:defining a primary combustion zonehaving an inlet for primary combustion air and an outlet for gaseouscombustion products, providing fuel to said primary combustion zone,delivering less than the stoichiometric amount of air to said inlet toburn said fuel at a temperature and under conditions whereby thecombustion products include significant residual amounts of unoxidizedhydrocarbons and carbon monoxide, defining a secondary combustion zonecoupled to the outlet of the primary combustion zone, conducting thestream of hot gaseous combustion products from the primary combustionzone to the secondary zone, the direction of gas flow from said primarycombustion zone to said secondary combustion zone defining a downstreamdirection, flowing said combustion gases around and between a pluralityof flow diverters in said secondary zone, and injecting combustionsupporting gas from downstream areas of each of said flow diverters andinto plural regions of said combustion gases as the latter passes aroundsaid diverters whereby to oxidize the residual unoxidized hydrocarbonand carbon monoxide at a temperature below which significant amounts ofnitrogen oxides are formed.
 2. The method set forth in claim 1including:a. controlling the total amount of combustion supporting gassupplied to said primary and secondary combustion zones so that saidtotal amount is substantially equal to the stoichiometric amount.
 3. Themethod set forth in claim 2 including:a. proportioning the total amountof said combustion supporting gas between said combustion zones in suchmanner that an increase or decrease in the air supplied to the secondarycombustion zone is accompanied by an inverse decrease or increase,respectively, of combustion supporting gas supplied to said primarycombustion zone.
 4. A method of reducing nitrogen oxides in the exhaustgases of combustion apparatus, comprising:a. burning fuel in a primarycombustion zone with substantially less than the stoichiometric amountof air, b. controlling the temperature of the burning reaction belowthat at which significant amounts of nitrogen oxides are producedwhereby substantial amounts of carbon monoxide and unburned hydrocarbonsare produced in the combustion products, c. dispersing into thecombustion products, before substantial heat is extracted therefrom,sufficient combustion supporting gas to oxidize substantially all of thecarbon monoxide and hydrocarbons in a secondary combustion zone, d.controlling the temperature of the reaction in the secondary combustionzone below that at which significant amounts of nitrogen oxides areproduced, e. controlling the total amount of combustion supporting gassupplied to said primary and secondary combustion zones so that saidtotal amount is substantially equal to the stoichiometric amount, f.proportioning the total amount of said combustion supporting gas betweensaid combustion zones in such manner that an increase or decrease in theair supplied to the secondary combustion zone is accompanied by aninverse decrease or increase, respectively, of combustion supporting gassupplied to said primary combustion zone, and g. regulating the amountof combustion supporting gas supplied to said combustion zones inaccordance with the carbon monoxide content of the exhaust gases fromthe secondary combustion zone.
 5. A method of reducing nitrogen oxidesin the exhaust gases of combustion apparatus, comprising:a. burning fuelin a primary combustion zone with substantially less than thestoichiometric amount of air, b. controlling the temperature of theburning reaction below that at which significant amounts of nitrogenoxides are produced whereby substantial amounts of carbon monoxide andunburned hydrocarbons are produced in the combustion products, c.dispersing into the combustion products, before substantial heat isextracted therefrom, sufficient combustion supporting gas to oxidizesubstantially all of the carbon monoxide and hydrocarbons in a secondarycombustion zone, d. controlling the temperature of the reaction in thesecondary combustion zone below that at which significant amounts ofnitrogen oxides are produced, e. controlling the total amount ofcombustion supporting gas supplied to said primary and secondarycombustion zones so that the total amount is substantially equal to thestoichiometric amount, f. proportioning the total amount of air betweensaid primary and secondary combustion zones, and g. sensing the carbonmonoxide content of the exhaust gases from said secondary combustionzone and increasing the combustion supporting gas supplied to thesecondary zone and reducing the combustion supporting gas supplied tothe primary zone in accordance with increased carbon monoxide beingsensed, and further, decreasing the supporting gas supplied to thesecondary zone and increasing the supporting gas supplied to the primaryzone in accordance with decreased carbon monoxide being sensed.
 6. Amethod of reducing nitrogen oxides in the exhaust gases of combustionapparatus, comprising:a. burning fuel in a primary combustion zone withsubstantially less than the stoichiometric amount of air, b. controllingthe temperature of the burning reaction below that at which significantamounts of nitrogen oxides are produced whereby substantial amounts ofcarbon monoxide and unburned hydrocarbons are produced in the combustionproducts, c. dispersing into the combustion products, before substantialheat is extracted therefrom, sufficient combustion supporting gas tooxidize substantially all of the carbon monoxide and hydrocarbons in asecondary combustion zone, d. controlling the temperature of thereaction in the secondary combustion zone below that at whichsignificant amounts of nitrogen oxides are produced, e. sensing thelevel of a constituent in the gases from said secondary combustion zone,and f. controlling the amount of combustion supporting gas in accordancewith the level of said constituent.
 7. The method set forth in claim 6wherein:a. said sensed constituent is carbon monoxide.
 8. The method setforth in claim 6a. wherein the sensed constituent in the gases from saidsecondary zone is combustible, and b. controlling the amount ofcombustion supporting gas supplied to said secondary combustion zone byincreasing said amount when said constituent increases and decreasingsaid amount when said constituent decreases.
 9. The method set forth inclaim 8 wherein:a. said sensed constituent is carbon monoxide.
 10. Themethod set forth in claim 6 and including the steps of flowing saidcombustion products around and between a plurality of flow diverters insaid secondary zone, and injecting said sufficient combustion supportingfrom the downstream areas of each of said flow diverters and into pluralregions of said gas stream as the latter passes around said diverters.